TUESDAY, Aug. 10 (HealthDay News) -- In certain situations, competing for a mate may shorten a man's life.

U.S. researchers found that when men reach sexual maturity in settings where they far outnumber women, they live an average of three months less than males from areas with a more equitable gender ratio.

While previous studies have examined gender ratios and longevity in animals, this is the first time it's been studied in humans, according to senior author Nicholas Christakis, a professor of medicine and medical sociology at Harvard Medical School, and a professor of sociology at Harvard University's Faculty of Arts and Sciences.

"At first blush, a quarter of a year may not seem like much, but it is comparable to the effects of, say, taking a daily aspirin, or engaging in moderate exercise. A 65-year-old man is typically expected to live another 15.4 years. Removing three months from this block of time is significant," Christakis said in a Harvard news release.

For this study, he and his colleagues analyzed data from the long-term Wisconsin Longitudinal Study, which followed people who graduated from Wisconsin high schools in 1957. The researchers calculated the gender ratios of each high-school graduating class and then looked at the life spans of the graduates.

As of 2007, men from classes with more boys than girls didn't live as long as those from classes that had a more even gender balance. For 65-year-old men, the difference in lifespan was 1.6%.

The study appears in the August issue of the journal Demography.

While the researchers didn't look into the reasons for their finding -- or prove that it was related to the struggle to find a mate -- they believe a number of social and biological factors play a role. Trying to find a mate is stressful, and stress is a known contributor to health problems, Christakis noted.